It is known to frost the outer surface of the arc tube of high pressure discharge lamps which are filled only with mercury partly or even entirely, for instance, by sandblasting with quartz sand. The purpose of frosting is to attain diffusion to provide a larger beam spread (see German Pat. No. 950,224, page 2, lines 84 and 85 and page 1, lines 5-12) or to render the arc which is contracted in operation, visually sufficiently broad so that the dark space between the arc and the tube wall is brightened and the electrodes are no longer discernible (see U.S. Pat. No. 3,384,771, claim 1, column 4, lines 2-5; column 3, lines 19-21; and column 2, lines 30-32).